Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!cs.utexas.edu!sun-barr!newstop!texsun!digi!kgallagh
From: kgallagh@digi.lonestar.org (Kevin Gallagher)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Can I Low Level Format my MFM drive with 17 < SECTORS < 26 ?
Message-ID: <1991Mar19.185406.29567@digi.lonestar.org>
Date: 19 Mar 91 18:54:06 GMT
References: <5832@trantor.harris-atd.com> <1991Mar16.022315.28142@digi.lonestar.org> <5855@trantor.harris-atd.com>
Organization: DSC Communications Corp, Plano, TX
Lines: 35

In article <5855@trantor.harris-atd.com> sonny@trantor.harris-atd.com (Bob Davis) writes:
>	But I can take an MFM drive and RLL it to 26 sectors/track and there
>is no problem with getting done with all 26 tracks in one revolution of
>the platter.

That's because MFM encoding of a sector takes up more time and space
than RLL encoding.  The platters in the drive spin at the same speed
regardless of how you format the drive.  The nature of RLL encoding
places greater demands upon the drive hardware.  However, it turns out
that the quality of a large number of drives manufactured and sold as
MFM drives are of the quality that then can reliably read and write
RLL encoded data.  The specs needed to reliably read RLL encoded data
are somewhat more demanding than those needed to read MFM encoded
data.

There are plenty of similar examples in the floppy disk world.  In the
early days of the Apple II computer, the first Apple drives had 13
sectors per track.  Later on, Apple realized that the drive could
reliably read data written to disk with a more economical encoding
scheme which would permit 16 sectors per track.  All then needed to do
was modify the controller to deal with the new encoding.

IBM first came out with floppies with 8 sectors per track and later
upgraded to 9 sectors per track on 5.25 inch drives.

And so on.  Thus, unless you slow down the fixed speed of the drive,
there is not enough room on a track to fit more than 17 MFM sectors
per track.  But slowing down the speed of the drive runs the risk that
the hardware will not be able to read what you write, especially on
the shorter length inside tracks.
-- 
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Kevin Gallagher        kgallagh@digi.lonestar.org OR ...!uunet!digi!kgallagh
DSC Communications Corporation   Addr: MS 152, 1000 Coit Rd, Plano, TX 75075
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